Orchard and other crops are subject to the danger of freezing in both spring and fall. In the spring, a late frost can ruin the buds; and in the fall, the mature fruit is damaged. Elaborate attempts have been made to prevent frost damage, including the use of smoke generators, heaters, and varieties of fans oriented to generate turbulence and keep the air moving. Stagnation of cold air seems to be the problem, as the heavier colder layers accumulate in most spots to a depth that can submerge orchard trees. Fans, and even helicopters, have been used to induce downward movement of the warmer upper air to displace and warm the lower air. These procedures have not been fully effective, as is indicated by the general availability of rather expensive frost insurance. The cost of this, or the alternate possibility of the loss from crop damage, establishes that equipment of substantial cost is justified if it is effective.